Decreasing world reserves and diminishing availability of crude oil have created considerable incentive for the development and use of alternative fuels. In recent years the ever increasing value of fossil hydrocarbon liquids and gases has directed research and development to the possibilities of employing bio-mass materials for fuel purposes. In particular, attention has been focused on fermentation derived ethanol for car fuel purposes. Ethanol is gaining wide popularity as such a fuel when mixed with gasoline to form a mixture known as gasohol. Automobiles can run on gasohol containing up to about 10 volume percent ethanol without requiring engine modifications. To prevent phase separation during storage, gasohol should contain at most only small amounts of water. Thus the ethanol that is mixed with the gasoline needs to be relatively dry. Desirably dry ethanol contains 1% or less by weight of water, preferably less than 0.5% water, and more preferably 0.3% or less of water.
One common method of ethanol production is the fermentation of mash, usually from corn and/or sugar cane. During alcoholic fermentation, sugar, particularly glucose, is converted into ethanol and carbon dioxide in the presence of yeast cells that contain the enzyme complex zymase. Glucose is produced by enzymatic splitting of maltose, which is itself formed during the hydrolytic, enzymatic splitting of starch or is developed during the manufacture of sugar. In addition to ethanol, the ethanol solutions developed during alcoholic fermentation contain soluble and insoluble components of vegetable cells and builders, yeast cells, starch and fractions of starch, various sugars, salts, and water. The ethanol content of the solutions obtained during alcoholic fermentation is usually about 6 wt. % and about 94% water. At higher alcohol levels, the bacteria begin to die and fermentation slows or ceases.
As described in the online Encyclopaedia Brittanica, rectification is the process of purifying alcohol by repeatedly or fractionally distilling it to remove water and undesirable compounds. A fermentation mixture primarily contains water and ethanol, and distillation involves increasing the percentage of ethanol in the mixture. Water vaporizes very easily, however, and, unless care is taken, the distillate of a fermentation mixture will contain unacceptably large quantities of water. The fermentation mixture furthermore contains small quantities of constituents such as, for example, organic aldehydes, acids, esters, and higher alcohols. The ones that remain in the product are called congeners, and the congener level is controlled by the particular rectification system and by the system's method of operation.
A multicolumn rectifying system commonly consists of three to five columns. The first column is a preliminary separation column called the beer still, or analyzer. It usually consists of a series of metal plates with holes punched in them and baffles to control the liquid levels on the plates. The product coming from this column is generally between 55 and 80 percent ethanol. A 95-percent product can be produced on a two-column system consisting of a beer column and a rectifying column.
Water cannot be completely removed from ethanol by distillation because of the formation of an azeotrope containing 95.5 wt. % ethanol and 4.5 wt. % water, which limits the upper concentration of ethanol that can be obtained by rectification regardless of the number of theoretical plates employed. Distillation processes have the further drawback that they require a large amount of energy. Special techniques are required to dehydrate ethanol beyond the 95.5 wt. % ethanol content level and typically require a considerable additional amount of energy. The high energy cost of ethanol separation by distillation is an economic impediment for using ethanol produced by alcoholic fermentation as an engine fuel.
Ethanol can also be produced by the catalytic hydration of ethylene. This reaction is often carried out in the presence of excess water with a catalyst such as phosphoric acid on clay. The resulting ethanol is not free of water.
Thus, although wet ethanol can be readily produced, new methods and apparatus are needed in order to economically separate water from ethanol. Wet ethanol refers to ethanol that contains water. Typically wet ethanol contains water in the range of 1% to 95% by weight, often in the range of about 2% to 50%, commonly in the range of about 3% to 30%, and frequently in the range of 4°/, to 10% by weight.
Pressure swing adsorption (PSA) is a gas separation process in which the adsorbent is regenerated by rapidly reducing the partial pressure of the adsorbed component, either by lowering the total pressure or by using a purge gas. In the original PSA cycles, invented by Skarstrom (1960), the two steps of adsorption and depressurization/purge are carried out in two adsorbent beds operated in tandem, enabling the processing of a continuous feed. Since introduction of the Skarstrom cycle, many more sophisticated PSA processes have been developed and commercialized. Such processes have attracted increasing interest more recently because of their low energy requirements and low capital investment costs.
The selectivity in a PSA process comes from differences in either adsorption equilibrium (equilibrium-controlled) or adsorption rate (kinetic-controlled) between the components to be separated. Although the overall performance of PSA depends on both equilibrium and kinetics, the relative importance varies for different applications (Ruthven et al., 1994). PSA is best suited for components that are not too strongly adsorbed. On the other hand, thermal swing adsorption (TSA) is preferred for very strongly adsorbed components, since a modest change of temperature produces a large change in gas-solid adsorption equilibrium. With better understanding of adsorbents and advances in their development for more efficient separation, there are many economic incentives for the further design and improvement of adsorption processes for gas separation.
In a PSA system, regeneration is achieved by first stopping the feed flow, then depressurizing the adsorbent, usually by passing regeneration gas through the bed counter-current to the feed direction. The regenerating gas is generally at a lower pressure than that of the air and is free of impurities. During the feed step, the adsorption process generates heat, which causes a thermal pulse to progress downstream through the bed. During the regeneration process, this same amount of heat must be supplied to desorb the impurities that have been adsorbed. In PSA, the aim is to commence regeneration before the heat pulse mentioned above has reached the downstream end of the bed. The direction of the heat pulse is reversed by the regeneration process, and the heat derived from the adsorption stage is then used for desorbing the impurities during regeneration, thereby avoiding the need to add heat during the regeneration step.
In the TSA process, the cycle time is extended, and the heat pulse is allowed to exit the adsorbent bed during the feed period. To achieve regeneration, it is therefore necessary to supply heat to desorb the material. To this end, the regenerating gas is heated for a period of time to produce a heat pulse that moves through the bed counter-current to the feed direction. This flow of heated regenerating gas is usually followed by a flow of cool regenerating gas that continues the displacement of the heat pulse through the bed toward the upstream end.
Each process has its own characteristic advantages and disadvantages. TSA is energy intensive because of the need to supply heat to the regenerating gas. Typically, there will be more than one unwanted gas component removed in the process, and generally one or more of these components will adsorb strongly and others much more weakly. The temperature used for regeneration in TSA needs to be sufficient for the desorption of the more strongly adsorbed component. The temperatures required for the regenerating gas are typically high enough, e.g., 150-200° C., to place demands on the system engineering which increases costs.
While the PSA system avoids many of the disadvantages associated with high temperatures, the short cycle time that characterizes PSA has its own consequences. In each cycle of operation, the adsorbent is subjected to a feed period during which adsorption takes place, followed by depressurization, regeneration and repressurization. During depressurization, the feed gas in the bed is vented off and lost, which is referred to as a “switch loss.” The short cycle time in the PSA system gives rise to high switch losses and, because the cycle is short, it is necessary that the repressurization be conducted quickly. This rapid repressurization causes transient variations in the feed and product flows, which can adversely affect the plant operation, particularly the operation of processes downstream from the adsorption system.
Derr, U.S. Pat. No. 2,137,605, the disclosure of which is incorporated herein by reference, describes a method of producing anhydrous and absolute alcohol that comprises passing alcohol vapors containing water vapor through a bed of freshly reactivated alumina moistened with liquid alcohol.
Oulman et al., U.S. Pat. No. 4,277,635, the disclosure of which is incorporated herein by reference, describes a process of concentrating relatively dilute aqueous solutions of ethanol by passage through a bed containing granules of crystalline silica polymorph such as silicate, which adsorb the ethanol. A displacer fluid containing at least 80 wt. % ethanol is continuously passed through the bed to displace residual dilute ethanol feed without displacing the ethanol from the granules, and then removing the adsorbed ethanol together with a portion of the displaces fluid.
Greenbank et al., U.S. Pat. No. 4,465,875 and Ginder, U.S. Pat. No. 4,407,662, the disclosures of which are incorporated herein by reference, describe the use of molecular sieves to dry ethanol.
Fornoff, U.S. Pat. No. 4,273,621, the disclosure of which is incorporated herein by reference, describes a process for dehydration of ethanol comprising distilling a crude aqueous ethanol feedstock to produce a gaseous ethanol-water mixture containing about 90 wt. % ethanol, drying the mixture in the presence of carbon dioxide with a crystalline zeolite 3A, and allowing the product ethanol to condense at ambient temperatures.
Umekawa, JP Appl. No. 59004415, the disclosure of which is incorporated herein by reference, teaches that the size of an air purification cylinder and the pressure drop within it can by reduced through the use in the adsorbent bed of a deep layer of large particles (3.2 mm), followed by a shallow layer of small particles (1.6 mm) of the same adsorbent. The bed size and pressure drop of this layered configuration are lower than for beds constructed solely of 3.2 mm or of 1.6 mm particles.
Miller, U.S. Pat. No. 4,964,888, the disclosure of which is incorporated herein by reference, describes a multiple zone adsorption process using larger particles in the equilibrium zone and smaller particles in the mass transfer zone (MTZ), which reduces the size of the MTZ and minimizes the pressure drop increase that would occur if only small particles were used in both zones. The process is applicable to the separation of hydrogen from a feed containing hydrogen and at least one other component selected from among carbon monoxide, methane, and nitrogen.
Garrett, UK Pat. Appl. GB 2 300 577, the disclosure of which is incorporated herein by reference, describes an adsorption apparatus suitable for the separation of nitrogen from a stream of compressed air that contains particles in the size range between 6 mesh and 12 mesh, which are deployed either in discrete layers or as a gradient of sizes, the largest particles being located near the feed inlet, and the smallest particles downstream near the outlet of the adsorber.
Batta, U.S. Pat. No. 3,564,816, the disclosure of which is incorporated herein by reference, describes a pressure swing process using at least four adsorption zones to separate, for example, CO, CO2, CH4, NH3, H2S, A, N2, and H2O from H2, and O2, N2, and CO2 from air.
Jones et al., U.S. Pat. No. 4,194,892, the disclosure of which is incorporated herein by reference, describes a rapid adiabatic pressure swing process useful for separating nitrogen from air, ethylene from nitrogen, and methane and/or carbon monoxide from hydrogen, the total cycle time being less than 30 seconds.
Reiss, U.S. Pat. No. 5,114,440, the disclosure of which is incorporated herein by reference, describes a process for oxygen enrichment of air by means of vacuum swing adsorption using adsorbers containing Ca zeolite A molecular sieves, the adsorbers of the VSA units being filled with separate layers of Ca xeolite A molecular sieves having different adsorption characteristics.
Hay et al., U.S. Pat. No. 5,176,721, the disclosure of which is incorporated herein by reference, describes an adsorber apparatus that is preferably used for the separation of oxygen from air and comprises a vessel containing an adsorbent mass separated into two longitudinal parts, the first part containing smaller particles and the second part containing larger particles.
Bloch, U.S. Pat. No. 3,474,023, the disclosure of which is incorporated herein by reference, describes an apparatus for continuous drying of a moist fluid such as hexane that utilizes a low DC potential to electrolyze the water adsorbed by a desiccant such as anhydrous calcium sulfate, thereby producing gaseous hydrogen and oxygen that can be separately collected.
Jean, U.S. Pat. No. 3,359,707, the disclosure of which is incorporated herein by reference, describes a method and apparatus for removing carbon dioxide and moisture from stale air, wherein the adsorbent bed is regenerated using high frequency electrical energy to effect dielectric heating of the adsorbent particles.
Koseki et al, U.S. Pat. No. 4,205,459, the disclosure of which is incorporated herein by reference, describes an apparatus for regenerating an absorbent in which adsorbed water is removed by heating in a furnace.
Mezey at al., U.S. Pat. No. 4,322,394, the disclosure of which is incorporated herein by reference, describes the separation of a gas mixture by selective adsorption of, for example, carbon dioxide and hydrogen sulfide from natural gas, using microwave heating to remove the adsorbed materials from saturated solid noncarbon adsorbents. Park et al., U.S. Pat. No. 6,634,119, the disclosure of which is incorporated herein by reference, describes an adsorptive drying apparatus in which microwaves are applied during the regeneration of an adsorbent.
Bonnissel, Luo, and Tondeur, Ind. Eng. Chem. Res., 2001, 40 (10), 2322-2334, 2001) describes a thermal swing adsorption process based on a composite adsorbent bed consisting of an arrangement of layers of activated carbon particles that are separated by sheets of a highly conductive graphite material. The process, which also makes use of thermoelectric devices (Peletier elements) to alternately heat and cool the adsorbent bed, is illustrated experimentally by the uptake and concentration of carbon dioxide from a helium flow.
Despite the technology described above, there remains a need to development new apparatus and methods to be able to economically separate water from ethanol on a large scale. In particular there is a need to produce ethanol that is nearly free of water.